The process of percussive tunneling drilling in subterranean formations involves lifting a drill sting, which is tipped with a drill bit, into the desired location of the subterranean formation. During the drilling process, the drill string (and therefore the bit) is forced against the material of the subterranean formation to cut away the material. The drill bit is hammered repeatedly against the material of the subterranean formation, while sometimes being rotated. An example of conventional percussive drilling equipment is illustrated in FIG. 1 which shows a drill string containing a drill bit 1, an extension body 2, a first drill rod 3, a drill rod connector 4, and a second drill rod 5. The drill bit 1 is repeatedly hammered against the bottom of the borehole to cut away the material and thereby increase the depth of the borehole.
One of the problems encountered in percussive tunneling drilling is drilling in a straight line, i.e., keeping the borehole as straight as possible. The drill bit tends to deviate from a straight line because, as illustrated in FIG. 1, there exists a gap between the guide and the borehole wall. But this gap unfortunately allows the drill bit to wobble from the straight axis in which the drilling should occur, causing the bit to wander and the borehole to therefore not be straight, as shown in FIG. 1. This problem occurs less with larger drill rods, as opposed to smaller rods, because the strength provided by their larger size allows less wobble.